robertsontshersby-harvie



May 1, 1956 R. B. ROBERTSON SHERSBY-HARVIE WAVE GUIDE ACCELERATOR SYSTEMOriginal Filed Oct. 11, 1949 2 Sheets-Sheet 1 v I 0 a] 0.2 QJQfQ-JQ60.7Qfi 0.91.011 1,215

nan 60105 i W WP QZZgfi INVENTOR m Mex-WWW MVEGl/IOE ATTORNEYS May 1,1956 Re. 24,146

R. B. ROBERTSON-SHERSBY-HARVIE WAVE GUIDE ACCELERATOR SYSTEM OriginalFiled Oct. 11, 1949 2 sheets-sheet 2 INVENTOR ROBE/PT BRUCE 2055x7250-.snsxassy-z-uzewe,

ATTORNEYS Re. 24,146 Reissued May 1, 1956 24,146 WAVE GUIDE ACCELERATORSYSTEM Robert Bruce Robertson-Shersby-Harvie, Buntingford, England,assignor to National Research Development Corporation, London, England,a British corporation Original No. 2,698,381, dated December 28, 1954,Serial No. 120,703, October 11, 1949. Application for reissue October28, 1955, Serial No. 543,639

Claims priority, application Great Britain October 18, 1948 4 Claims.(Cl. 250-27) Matter enclosed in heavy brackets appears in the originalpatent but forms no part of this reissue specification; matter printedinitalics indicates the additions made by reissue.

This invention relates to electrical particle or ion accelerators inwhich moving charged particles such as ions or electrons are acceleratedunder the influence of a travelling electromagnetic wave, of phasevelocity substantially equal to the particle velocity. In suchaccelerators, generally referred to as travelling-wave linearaccelerators, the propagation path for the wave and the path of themoving particles, which will hereinafter be referred to as ions, isusually arranged in a substantially linear form, as distinct from theclosed-loop form of the ion path in such particle acceleration devicesas the synchrotron.

A travelling wave linear accelerator usually consists of ahigh-frequency source feeding power into a waveguide accelerating tubewhich is terminated by a matched dummy load which absorbs withoutreflection the highfrequency power remaining at the output end of thewaveguide tube. The waveguide will be so arranged by suitabledimensioning, loading or corrugation of the waveguide inner surface thatthe phase velocity of the electromagnetic wave propagated down the guideis suitable for coupling to moving ions to occur. For acceleration atvelocities below the relativistic region the properties of the guide maybe so varied along its length that the phase velocity of the wave is afunction of position along the guide in order that the travelling waveand accelerating ions may be kept in phase synchronism.

In the following description and claims, it is to be understood that theterm waveguide relates to a structure having a boundary surface andwherein an electromagnetic wave is supported in the space adjacent saidboundary surface by means of circulating currents in the boundarysurface.

The following description will be given with reference to theaccompanying drawings in which:

Fig. 1 is a diagram showing a number of explanatory curves.

Fig. 2 is a diagrammatic representation of an arrangement illustratingthe basic principles of the invention.

Fig. 3 is a diagrammatic representation similar to Fig. 2, illustratingin more precise form one possible arrangement according to theinvention.

Fig. 4 is a diagrammatic representation of a part of the arrangementshown in Fig. 3, and

Fig. 5 illustrates a complete linear electron accelerator installationembodying the invention.

The energy of an ion at the output end of a travellingwave linearaccelerator, expressed as an equivalent voltage, is given by theintegral, over the length of the accelerator, of the peak acceleratingfield. It is thus apparent that the output energy is, to a firstapproximation, proportional to the accelerator length, but thatattenuation of the travelling wave eventually oflsets the effect ofincreased length. From the point'of view of R. F. power economy the bestaccelerator would obviously have such a length that the R. F. powerremaining at the output end was negligible; the provision of such anaccelerator would, however present practical difficulties and mayinvolve an accelerator of undue length. A figure of merit may beobtained for an accelerator design which is given by:

where V is the output ion energy, W is the power supplied by the R. F.source and L is the accelerator length. If this factor is plottedagainst accelerator length a curve such as that indicated at 1 in Fig. 1of the accompanying drawings is obtained. The curves of Fig. 1actuallyillustrate the relation between the relative efliciency, theratio of r to an optimum efliciency against the ratio of actual length Lto the length Lo for optimum efli ciency. As the length is increasedfrom zero, the efliciency factor 1 0 increases to an optimum value;below this length the accelerator is insufliciently long to make optimumuse of the R. F. power. As the length is increased beyond the optimumthe efliciency" factor falls as the increasing attenuation partiallyoffsets the effect of increasing length upon the output energy V. It canbe shown that the optimum value of 1; corresponds to a length L0 havingan attenuation of 1.25 nepers (approximately 11.08 db), so that with anaccelerator of optimum length only about 10% of the R. F. power iswasted.

In practice considerations of frequency stability and constructionaltolerances generally prevent linearaccelerators being made as long asthe optimum length. It will be obvious that as the length of thewaveguide employed to propagate the travelling wave is increased, thedeparture of the frequency of the R. F. power supply from the assignedvalue which can be tolerated before the travelling wave and moving ionsbecome out of phase at the output end of the accelerator is reduced. Inother words the bandwidth of the accelerator becomes so narrow as itslength is increased that the frequency of the R. F. source cannot easilybe maintained within the band; Errors in dimensions of the waveguide anderrors in the dimensions and positioning of any corrugations or otherloading elements in the waveguide operate, in the same manner as changesin frequency, to cause the wave and ions to get out of phase, and it isapparent therefore that as the accelerator length is increased thepermissible tolerances on dimensions of the accelerator structure maybecome impossibly small. As a result of these factors practicaltravelling-wave linear accelerators are made considerably less than theoptimum length with the result that a considerable fraction of the R. F.power supplied is wasted in the dummy load.

The low poWer-efliciency inherent in practical designs oftravelling-Wave linear accelerators would be of no importance from apurely economic point of view based on running cost; however, the sizeof R. F. power supply available is limited and considerable diflicultytherefore exists in obtaining the high peak fields required to produceions of energies above a certain level with the practicably achievablelengths of accelerator.

It is an object of the present invention to provide a travelling-wavelinear accelerator arrangement which is of less than the optimum lengthand in which the figure of merit" is improved over the value obtainablewith known arrangements, by utilization of the R. F. power available atthe output of the waveguide accelerating tube to reinforce the R. F.power supplied, instead of dissipating the output power in a dummy load,so that for a given size of R. F. power supply the peak value of theaccelerating field is increased.

It is a further object of the invention to provide a to manufacture thanknown travelling-wave-linear accelerators of comparable performance andwhich is capable of operating over a band of frequency, comparable withthe bandwidth obtainable with linear accelerator'systems which are ofshort length and have a relatively low performance, while retaining'arelatively high performance.

In-considering how-to make use of the R. F. power which would normallybe wasted in a straight-forward travelling-wave linear accelerator, itwould appear that anarrangement such as that illustrated in Fig. 2 woulden able all the R. F. power supplied to be utilized. The R. F. powersource 1 is coupled to the input of the waveguide 2, which supports thetravelling wave, by means. of a bridge 3. Power is fed back from theoutput end of the waveguide to the bridge where it is combined with thepower from source 1. The directions of power flow are indicated byarrows and it will be apparent that the power flux in the acceleratorwaveguide is greater than that supplied from the source; in this respectthe system is similar to a resonant system.

A non-dissipative bridge may be designed, composed of suitablewaveguidecircuit elements, which will satisfy the steady state power conditionsof Fig. 2 but such a bridge cannot be designed which will also satisfythe conditions necessary to secure stability. The stable state ofthesystem illustrated in Fig. 2 requires power to circulate in bothdirections round the loop including the accelerator waveguide, thesystem then functioning as a resonant system. In the arrangementaccording to the invention the system is stabilized by introducingdissipative elements into the bridge; one such power sink is sufficientand the accelerator arrangement may be as illustrated in Fig. 3 in whichthe dissipative element 4 of the bridge is indicated as being externalto the bridge 3 and connected as the load on one of the four outlets a,b, c, d from the bridge, which may be regarded as being non-dissipativein itself.

The detailed design of the bridge 3 is defined by stipulating the steadystate power ratios relating the power from the source (Wb), thefeed-back power (We), the power fed to the accelerator waveguide (We)and the power dissipated in the absorbing load 4 (We); also the arms aand b are mutually conjugate, i. e., there can be no power transferencebetween them. The conjugate relationship of arms a and b of the bridgesystem ensures that; firstly, all the power flows through theaccelerator waveguide in the desired direction, and secondly; that theimpedance of the input arm b is independent of the amount of power fedback into arm a. This latter condition ensures that the input impedanceof the complete system remains constant during the period after application of power while the circulating power is being built-up in thesystem.

The power ratios of the bridge system must obviously be designed to fitthe attenuation of the accelerating waveguide and if the ratio of powerentering the waveguide to power leaving the waveguide is taken to be(1+n)/n, where n is determined by the attenuation in the guide, then thesteady-state power levels will be as indicated in Fig. 3. The powersupplied from the generator will be Wu, the fed-back power Wu. willequal nWb and the power entering the waveguide willbe (n-i-l)Wb. We willbe zero. It will be seen that if the special case is selected in whichn=l then the generator power and fed-back power are equal. In this casethe bridge system may be realised by straight forward known waveguidecircuits such as the hybrid-T joint (or magic-T) or the so-calledrat-race" junction. If a hybrid-T junction is employed as the bridgesystem for the special case of 2:1 ratio of input to output power in theaccelerator waveguide the T-junction will be formed of waveguides, allof the same cross-section, the generator and feed-back path beingcoupled to the E- and H-plane T-junction arms while the inline sectionsfeed the accelerator waveguide and the dummy load respectively.

Alternatively an arrangement of this latter kind may be employed withthe input and output arms interchanged. It will readily be understoodthat such an arrangement, in the steady-state condition and with thecorrect phase shift in the feedback path,v will cause the waves enteringthe two T-junction arms to add in phase in one direction (that going tothe accelerator) while in the reverse direction (feeding the dummy load)the two waves will cancel. Similar considerations may be applied to theemployment of a four-armed 'rat-race" junction under the specialcondition of n=1.

The enhanced efiiciency of figure of merit of the ac- .celerator systemincorporating power feedback results from the power flux through theactual accelerator waveguide being increased l+n times. Thus if 715 isthe steadystate figure of merit for the accelerator system with feedbackthen the relative figure is given by:

where n and no refer to the actual and optimum figures of merit of thesame accelerator waveguide without feedback. The factor (I1+1)='Wd/Wb isa measure of the power magnification of the system and is a function ofthe length of the accelerator waveguide, diminishing as the lengthincreases. The manner in which this factor varies with the relativelength L/Lo of the accelerator is indicated in curve 2 of Fig. 1.

Curve 3 of Fig. 1 indicates how, assuming no loss in the feedback path,the figure of merit 1 varies with the relative length of theaccelerator. It will be seen that the feedback efiiciency decreases asthe'length is increased, approaching the value obtained withoutfeed-back as the length increases. With short accelerator lengths thesystemincorporating feedback canyield efliciencies or figures of meriteven greater 1 than the value obtainable with the optimum length withoutfeedback.

Although the loss in the waveguide constituting the feedback path willingeneral be small compared with the forward loss in the acceleratingwaveguide, other loss will occur in practice in the feedback path andwill modify the efliciency. If constant attenuation is assumed in thefeedback path, such as might be caused by losses in the couplings, thenthe efiect clearly becomes more important as the accelerator length isreduced because the circulating power is increased. If the bridge systemis correctly proportioned to takeaccount of the loss in the feedbackpath, the'practical relationship between relative factor of merit andrelative length will be as shown in curve 4 of Fig. l, which indicatesthe relationship for an attenuation in the feedback path ofapproximately half a decibel.

One practical embodiment of the bridge system of-the completetravelling-wave linear'accelerator system of the invention isillustrated 'in Fig. 4 for the general case in which the input/outputpower ratio (n+1)/n for the accelerating waveguide, has a value otherthan 2. The bridge system ofFig- '4 is a; modified rat-race junctionarranged with the dimension of the various sections of waveguideadjusted to provide the required impedance match and power distributionconditions.

All the waveguide sections of Fig. 4 are of rectangular cross-section,supporting Ho. mode waves, the narrow dimensions of the guidecross-sections being in the plane of the paper. The length measuredaround the annular waveguide is 6M4 andthe four arms ad (correspondingto the bridge connections ad of Fig. 3) form series-T or E-plane Tjunctions spaced around the circumference of the annular guide as shown.

For transmission to occur through an E-plane T junction the two wavesentering or leaving the junction are in anti-phase. Thus ifpower fromthe R. F. source is fed in at armb it will'appear from arm d, which maybe utilized to feed the input end of the accelerator waveguide, but nopower will be supplied to arm a, to which the feedback connection fromthe remote end of thewaveguide is taken. Power is ableto pass from arm bto armc which is terminated by the matched dummy load (4 of Fig. 3). Thefed-back power entering the bridge via arm a is able to flow to the armd but not to the conjugate arm b. Power is able to flow also from thearm a to the arm c. Arm d may alternatively be located in the place e.Under the steady-state conditions of operation the waves reaching arm cfrom the arms b and a are arranged to cancel by adjustment of theiramplitudes by suitable proportioning of the waveguide impedances.

The semicircular portion of the waveguide annulus between arm a and theplane e, which carries the arms b and c is made with a uniform totalcharacteristic resistance Z1, while the other semicircular portion ismade with two portions of total characteristic resistances 2.2 and Zp asindicated.

If the impedance of the matched load fed by arm c and also the totalcharacteristic resistance of the waveguide section c is Zn, and also thearm d of characteristic resistance Z4, is matched to the input of theaccelerating waveguide, for correct operation of the bridge with nopower entering arm c the following relations must hold:

The characteristic resistances of the different sections of the annularguide and the different T arms are adjusted to the appropriate values bysuitable variations in the guide dimensions in the directions of the Evectors, as shown. The input impedances presented by the arms a and bare then given by:

The method of achieving the desired bridge conditions, described abovewith reference to Fig. 4, is given by way of example only. Otherwaveguide circuits may be devised to provide the necessary transmissionconditions; for example the hybrid-T junction, which was previouslydescribed as applied to the special case when n=l, may be modified byadjustment of the characteristic resistances of the various arms tofulfill the conditions required for some other arbitrary value of n in amanner similar to that described for the rat-race junction.

One of the efl'ects of the reduction in length of the acceleratingwaveguide which is possible with the accelerator system embodyingfeed-back is that the bandwidth of the waveguide is increased in thesame proportion as the reduction in length. However, when the feedbackis operating'a change of frequency will cause a reduction in circulatingpower because the generator power and fed-back power will only beproperly in phase at one predetermined frequency. This effect may beovercome by introducing a phase changer or line-lengthener into thefeed-back path, which may conveniently be adjusted, manually orautomatically, to null or minimise the power in the arm c of the bridge.If this adjustment is made the bandwidth of the accelerator system willbe determined entirely by the accelerating waveguide and will be thesame as if feedback were not used. A short accelerating guide may thusbe used to achieve a wide bandwidth, the performance being retained byfeedback.

Fig. 5 illustrates, diagrammatically, the linear electron acceleratorapparatus embodying the invention. The

arrangement shown comprises an accelerator waveguide provided withinternal corrugations which serve to retard the phase velocity of theradio frequency wave. launched in the waveguide so that coupling maytake place between the radio frequency wave and a stream of electronsfired axially down the waveguide by means of an electron gun 11. Thewaveguide is enclosed in a vacuum chamber 12 continuously evacuatedthrough an exhaust tube 13. Focussing coils 14, 15, 16 and 17 enable theelectron stream to be maintained axially throughout the guide.

Radio frequency energy from a magnetron oscillator 18 is fed to theaccelerator waveguide through a supply waveguide 19, a rat-racewaveguide bridge assembly 20 and an input waveguide 21 suitably matchedto feed into the waveguide 10.

At the output end of the waveguide 10 a feedback waveguide 22 is matchedto the waveguide 10 to receive the radio frequency energy from it andapply this energy through a variable phase control 23 to an arm of theratrace assembly 20. The fourth arm 24 of the rat-race assembly 20 iscoupled to an absorptive load, for example a water load.

I claim:

1. Electromagnetic waveguide structure comprising a rat-race waveguideassembly having a pair of conjugate input arms and a pair of outputarms, a utilisation waveguide connected to one of said output arms, afeedback waveguide connected between the output of said utilisationwaveguide and one of said input arms, a source of wave energy connectedto the other of said input arms and power dissipative means connected tothe other of said output arms, the output arm connected to saidutilisation waveguide being positioned at a point on said rat-race atwhich the inputs from said input arms combine to feed said utilisationwaveguide, the output arm connected to said dissipative means beingpositioned at a point on said rat-race at which the inputs from saidinput arms cancel one another.

2. Electromagnetic waveguide structure as claimed in claim 1 whereinsaid rat-race" comprises a closed loop having a first section ofcharacteristic impedance Z1, a second section of characteristicimpedance Z: and a third section of characteristic impedance Zp, a firstinput arm of characteristic impedance Zr, and a first output arm ofcharacteristic impedance 20 coupled to spaced points on said firstsection, a second input arm of characteristic impedance Zn. coupled tothe junction between said first and second sections and a second outputarm of characteristic impedance Zd coupled to the junction between saidsecond and third sections, said characteristic impedances Z1, Z1, Zp,Zn, Zb, Zc, and Zd being correlated by the formulae:

and0 Z n being an integer.

3. In a linear waveguide electron accelerator, radio frequencyenergising means comprising a source of radio frequency energy,- abridge assembly connecting said source to the radio frequency input ofsaid electron accelerator, a feedback waveguide connecting the radiofrequency output of said electron accelerator to said bridge assemblyand phase control means in said feedback waveguide, said bridge assemblybeing in the form of a ratrace comprising a closed loop having a firstsection of characteristic impedance Z1, a second section ofcharacteristic impedance Z2 and a third section of characteristicimpedance Zp, a first input arm of characteristic impedance Zb and afirst output arm of characteristic impedance Zs coupled to spaced pointson said first section, a second input arm of characteristic impedanceZn. coupled to the junction between said first and second sections and asecond output arm of characteristic impedance Z4 coupled to the junctionbetween said second and third sections,

7 said characteristic Z1, 22, 21 Zia, Z6, Z1: and Z4 being correlated bythe formulae:

and 0 Z n being an integer.

4. Electromagnetic waveguide structure comprising a non-dissipativewaveguide bridge assembly having a pair of conjugate input arms and apair of output arms, a

utilisation waveguide connected to one of said output arms, a feedbackwaveguide connected between the output of said utilisation waveguide andone of said input arms, a source of wave energy connected to the otherof said input arms and power dissipative means connected to the other ofsaid output arms, the output arm connected L to said utilisationwaveguide being positioned at a point on said bridge at which the inputsfrom said input arms combine to feed said utilisation waveguide, theoutput arm connected to said dissipative means-being positioned at: apoint on said bridge atwhich the inputs from said input arms cancel oneanother in the steady state condition.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS 2,147,454 Morton Feb. 14, 1939 2,284,751 LinderJune 2, 1942 2,367,295 Llewellyn Jan. 16, 1945 2,369,268 Trevor Feb. 13,1945 2,436,828 Ring Mar. 2, 1948 2,445,895 Tyrrell July 27, 19482,545,595 Alvarez Mar. 20, 1951 2,580,007 Dohler et a1. Dec. 25, 19512,641,731 Lines June 9, 1953 2,653,270 Kornpfner Sept. 22, 19532,667,597 Bailey Jan. 26, 1954 FOREIGN PATENTS 117,561 Australia Oct.14, 1943 628,570 Great Britain Aug. 31, 1949

